Automatic Transfer Switch (ATS) is an electric device that switches one or more load circuits from a power supply to another power supply, and is widely used in various situations. Because the main switching component of the ATS is a mechanical switch, the switching speed is slow (about 20 ms) and it is easy to cause an arc. The arc may cause a high temperature, ignite and vaporize the metal contact of the mechanical switch, and thus greatly reduce the life time of the switch.
Static Transfer Switch (STS) is an electric device that switches one or more load circuits from one power supply to another power supply, and is used in various situations having high requirements for switching speed. The main switch component of STS is a thyristor. Although the switching speed of thyristor (less than 3 ms) is greatly increased over ATS, due to the fact that the thyristor is a semiconductor rather than a conductor, the thyristor has a turn-on voltage drop that is much higher than that of a mechanical switch made of conductor. Thus, the turn-on loss is increased. Moreover, the cost of high-power thyristor is high, and the product cost is also greatly increased. The STS is even more expensive than an uninterruptable power supply (UPS) with the same capacity.
For a long time, it is an urgent requirement that a switch has a low turn-on loss, a fast switching speed, a long life time, a good protection for load during failure and a low price.
Part of the above requirements may be satisfied by simply connecting a thyristor and a mechanical switch in parallel, but this is only applicable for situations with low power. For example, when the current in the application is 1 kA or higher, the product cost becomes very high due to the high power requirement for the thyristor. Meanwhile, when a short-circuit failure occurs on the thyristor, the power supply directly supplies power to a load via the thyristor branch, and the mechanical switch is bypassed. Thus, a threat is caused for the load. A patent in which a thyristor and a mechanical switch are simply connected in parallel as described above is filed with USPTO as early as in 1984, however, no corresponding product can be found today though about 20 years past. Thus, the deficiency and shortcoming of the patent may be reflected.